1. Field of the Present Invention
The present invention relates to a polarizing plate useful for liquid crystal displays, especially, TN-type liquid crystal displays, and relates to a TN-type liquid crystal display including the polarizing plate. In particular, the present invention relates to a technique which reduces the occurrence of light leaks from the periphery of the display flame at the originally black display time in liquid crystal displays in high-temperature environments.
2. Description of the Related Art
Various types of liquid crystal displays, such as twisted nematic (TN)-type, vertical alignment (VA)-type, and in-place-switching (IPS)-type liquid crystal displays, have been widely used in monitors of personal computers (PCs) and television sets. In order to provide a wide view angle in each type, techniques have been proposed, which uses retardation films having various optical characteristics as view angle compensation films.
In general, a polarizing plate used in liquid crystal displays includes a polarizer having a light-polarizing function, a protective film attached to one side of the polarizer with an adhesive layer therebetween, and a retardation film (or a protective film in some cases) attached to the other side of the polarizer. Polyvinyl alcohol (hereinafter also referred to as PVA) is mainly used as a material of the polarizer. The polarizer is formed by uniaxially stretching a PVA film and then dyeing the stretched PVA film with iodine or dichroic dye, or dyeing a PVA film and then stretching the dyed PVA film; and cross-linking the resulting PVA film with a boron compound.
A material mainly used for the protective film is cellulose triacetate (hereinafter also referred to as TAC), which has the following advantages: high optical transparency, small birefringence, high surface smoothness, and high adhesion to polarizers of a saponified PVA.
A known cellulose acylate retardation film is prepared, for example, by laminating a cellulose acylate substrate and an optically anisotropic layer composed of a liquid crystal composition, or by stretching a cellulose acylate film. Other retardation films composed of norbornene resins or polycarbonate resins have also been variously proposed, in addition to the cellulose acylate films.
Improvements in display quality have been demanded with the spread of liquid crystal displays. Expanding applications have generated further demands for durability under severe conditions such as high temperature and high humidity. It is known that liquid crystal displays used for long time under severe conditions suffer from brightness unevenness appearing at black display sites on a screen due to light leakage. A solution to this problem has been strongly needed. Moisture absorption with time causes the contraction of the polarizer, resulting in deformation of the protective film and cells. The deformed protective film and cells cause stress to be applied to any of the components used, which generates phase difference. It is believed that this phase difference leads to the brightness unevenness, and the moisture absorption through the polarizing plate therefore is required to be reduced for the solution to such a problem. For example, JP-A2008-107499 discloses a polarizing plate which includes a protective film having a moisture permeability lower than a predetermined level and discloses a technique to suppress the dimensional change of the polarizing plate.